Pentafluoroethane is a hydrofluorocarbon (HFC), a useful alternative of chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC). It is widely used as low-temperature refrigerants, foaming agents, propellants and etching gases.
HFC contain substances having a global warming potential (GWP) several thousand times greater than carbon dioxide gas. This problem has been pointed out recently and HFC emissions have been increasingly controlled.
The emissions of pentafluoroethane will be generally attributed to:
(1) used refrigerants in scrapped refrigerators and the like; and
(2) exhaust gases from the manufacturing process of semiconductors.
In the case (1), pentafluoroethane, which is emitted as used refrigerants when refrigerators are scrapped, is a condensable liquefied gas that is a mixture of pentafluoroethane and refrigerant oil. It maybe liquefied by compression or cooling and be easily recovered in a closed vessel.
In the case (2), however, exhaust gases from the semiconductor manufacturing are often mixtures of pentafluoroethane and large amounts of non-condensable diluent gases. Recovering pentafluoroethane from such mixed gases by the method in the case (1) is difficult.
The production of pentafluoroethane often generates pentafluoroethane diluted with large amounts of non-condensable gases, in which case it is difficult to recover pentafluoroethane. Such mixed gases are therefore frequently decomposed or combusted instead of recovering pentafluoroethane, but these treatments are not economical. Various methods are proposed for recovering pentafluoroethane that is diluted with large amounts of non-condensable gases.
Membrane separation is one of such methods proposed. JP-B-H02-48529 discloses separation using a permselective composite membrane, which includes a porous support membrane and an active thin membrane formed thereon by crosslinking a crosslinkable silicone resin. Japanese Patent No. 3470180 discloses separation using a gas permeation membrane that comprises a polymer based on poly(4-methylpentene-1). JP-A-2003-190744 discloses separation with a separation membrane that is obtained by carbonizing a polymer. JP-A-2001-510395 discloses separation using an inorganic molecular sieve membrane. However, these methods entail complicated processes because of short membrane life, time degradation of membranes which causes intricate controlling of separation conditions, and the need of eliminating beforehand membrane-degrading components in the gas to be treated. Moreover, separation membranes should be exchanged frequently to maintain a sufficient separation performance, increasing the cost.
Activated carbons are known to adsorb organic substances in exhaust gases. However, the existing adsorbents do not have an adequate adsorption capacity enough for recovering pentafluoroethane. To reuse pentafluoroethane that is recovered, it should be released from the adsorbent, which requires large-scale facility with heating and cooling functions and large amounts of energy to cause economical disadvantages.
Recovering organic substances by absorption in solvents is proposed. JP-A-H02-40216 discloses an absorbent composition based on polyethylene glycol dialkyl ether. After pentafluoroethane is recovered using this absorbent composition, however, to reuse pentafluoroethane that is recovered, it should be separated from the absorbent composition. As is the case for recovering with the adsorbents, this purification requires large-scale facility with heating and cooling functions and large amounts of energy to cause economical disadvantages. JP-A-2000-117051 discloses a process in which a chemically stable fluoride that is a gas at room temperature is recovered by being dissolved in a fluoride that is liquid at room temperature. This process requires purification for separating pentafluoroethane, and the fluorides that are liquid at room temperature are very expensive. JP-A-2002-13872 discloses a process in which a diluted gas containing a fluorocompound is brought into contact with a low-temperature liquid at not more than −101° C. to recover the fluorocompound. However, cooling a solvent to such extremely low temperatures of −101° C. or below requires a large amount of energy, and the fluorocompound recovered by this process has a low concentration and needs a further treatment.